As an electric-discharge-machining power supply apparatus for supplying electric power to a machining gap between a machining electrode and an object to be machined by interrupting every predetermined downtime a pulse train that is turned on and off at predetermined timing, to perform electric discharge machining, an, electric-discharge-machining power supply apparatus utilizing charge and discharge of a capacitor as illustrated in FIG. 14 has been known.
In the diagram, a variable voltage DC power supply 1 supplies machining power between the machining electrode 2 and the object 3 to be machined, disposed in machining fluid and facing each other with a very small gap. In accordance with a switching-device driving pulse signal 8a from a pulse generating circuit 8, when the pulse signal 8a is high, a switching device 4 is turned on, so that voltage is applied between the electrode 2 and the object 3 to be machined, and when the pulse signal 8a is low, the switching device 4 is turned off, so that applying the voltage between the electrode 2 and the object 3 to be machined is halted.
In addition, the pulse generating circuit 8 operates based on pulse generating conditions such as pulse on/off time, the number of pulses in a pulse train, and downtime between pulse trains, from a pulse generating condition setting unit 9 such as an NC device.
FIGS. 15, 16, and 17 are diagrams illustrating relations between pulse trains from the pulse generating circuit 8, an inter-electrode voltage, and an inter-electrode current.
FIG. 15 illustrates a state in which no electric discharge has occurred because the gap between the machining electrode 2 and the object 3 to be machined is large. While the switching device 4 is on, a capacitor 6 is charged by a time constant determined almost by the resistance value of a current-limiting resistor 5 and the capacitance of the capacitor 6, and while the switching device 4 is off, the electric charge in the capacitor 6 is discharged through a discharging resistor 7 by the time constant determined by the resistance value of the discharging resistor 7 and the capacitance of the capacitor 6.
As illustrated in the figure, because the resistance value of the discharging resistor 7 is determined to be large enough compared with the resistance value of the current-limiting resistor 5 for charging, even if the switching device 4 is turned off, the electric charge in the capacitor 6 does not run down immediately, so that the voltage across the gap between the machining electrode 2 and the object 3 to be machined is increasing step by step, such as the first pulse, the second pulse, and the like in the pulse train, up to around the voltage V1 of the DC power supply 1.
When the pulse train is terminated, a pulse downtime starts, and the inter-electrode voltage is gradually going down close to 0 V. When the pulse downtime of a predetermined time period elapses, the next pulse train is generated.
FIG. 16 illustrates a state in which electric discharges have occurred in portions A, B, and C.
The portion A indicates a state in which an inter-electrode dielectric breakdown has happened so that an electric discharge has occurred, while the voltage is rising during the third pulse in the pulse train. The discharge current value at that time is the sum of a discharge current due to the electric charge charged in the capacitor 6 and a charging current flowing from the DC power supply 1 through the switching device 4, the diode 10, and the charging current-limiting resistor 5.
The portion B indicates that a state in which an inter-electrode dielectric breakdown tends to occur persists after the portion A, whereby an electric discharge has occurred in succession to the portion A. The discharge current value at this time is a little lower than the discharge current value during the portion A, because an electric discharge has occurred when the charged voltage of the capacitor 6 is a little lower. The portion C indicates that, after the electric discharge in the portion B, the inter-electrode voltage has been gradually increasing, and then a dielectric breakdown has occurred, to cause a discharge current to flow.
FIG. 17 illustrates a case in which the inter-electrode gap is narrow and short-circuited. While the machining electrode 2 and the object 3 to be machined are short-circuited, a short-circuit current determined by the voltage V1 of the DC power supply 1 and the resistance value of the current-limiting resistor 5 flows between the machining electrode and the object as an inter-electrode current every time the switching device 4 is driven by the pulse generating circuit 8.
FIG. 18 is a diagram illustrating the configuration of an electric-discharge-machining power supply apparatus of a type in which AC pulse voltage is applied to the machining gap between the machining electrode and the object to be machined.
In addition to the configuration of the DC-pulse type power supply apparatus illustrated in FIG. 14, a DC power supply device 17, a switching device 4a composed of a MOS-FET or the like, a current-limiting resistor 5a, and a diode 10a are included.
In the meantime, FIGS. 19, 20, and 21 are diagrams illustrating relations between pulse trains from the pulse generating circuit 8, the inter-electrode voltage, and the inter-electrode current. The driving pulse signal 8a for the switching device 4 is generated so as to include a predetermined number of pulses at a predetermined on/off time, and after a predetermined downtime, the pulse signal 8b for driving the switching device 4a is generated so as to include the same number of pulses as in 8a. The pulse train 8a and the pulse train 8b are alternately repeated at the predetermined downtime.
A similar electric-discharge-machining power supply apparatus of an AC pulse type in which voltage of both positive and negative polarities is applied to the gap between the object to be machined and a machining electrode is disclosed, for example, in Japanese Patent Laid-Open No. 55117/1991.
FIG. 19 illustrates a state in which no electric discharge has occurred because the gap between the machining electrode and the object to be machined is large. While the pulse signal 8a is being outputted, the inter-electrode voltage is saturated around V1. Then after the following downtime, while the pulse signal 8b is being outputted, the inter-electrode voltage is in the reverse polarity and saturated around V2.
FIG. 20 illustrates a state in which electric discharges have occurred in portions A, B, C, and D. The polarity of the discharge current in the portion D is obviously opposite to those in the portions A, B, and C.
FIG. 21 illustrates a state in which the machining electrode and the object to be machined are short-circuited. The polarity of the short-circuit current is also reversed every time the polarity of the inter-electrode voltage is reversed.
Patent document 1: Japanese Patent Laid-Open No. 55117/1991